1. Field of the Invention
The present invention concerns delay units of the type used for time-controlled initiation of energetic materials, for example, delay units of the type used in delay detonators, and methods of making such delay units.
2. Related Art
Conventional pyrotechnic delay units comprise a pulverulent pyrotechnic composition encased within a soft metal tube, such as a tube of lead or pewter. Such conventional delay units are typically placed within a detonator shell between the input signal from a fuse, such as shock tube, and the explosive output charge of the detonator. Detonation of the output explosive charge is delayed by the time it takes the length of pyrotechnic material to burn from its input to its output end. As is well known to those skilled in the art, it is necessary to very closely control the delay periods of individual detonators; typical delay periods range from 9 to 9,600 milliseconds or more, for example, 9, 25, 350, 500 and 1,000 milliseconds. Attainment of consistently accurate and precise delay times by burning of a column of pyrotechnic material is inherently limited, and the art is assiduously developing electronic delay units in order to increase delay time accuracy, despite the increased cost of electronic delay units as compared to pyrotechnic delay units.
International Application WO 2004/106268 A2 of Qinetiq Nanomaterials Limited for “Explosive Devices”, published 9 Dec. 2004, discloses explosive devices printed onto substrates from inks which may contain particles as small as 10 micrometers in diameter “for even . . . 0.1 micrometer or less in diameter.” (Page 4, lines 18-24.) Figures such as FIGS. 1 and 2 disclose serpentine or spiral patterns of printed explosive ink on a substrate. For example, there is described at page 15, lines 11-29, printing of the explosive ink in a single line which starts adjacent a heating element and terminates adjacent a secondary explosive material. The printed line of explosive ink initiates the secondary explosive. A zig-zag pattern may be used and will increase the delay time provided by the device.
The use of nanoporous iron oxide as the oxidizer component of propellants, explosives and pyrotechnic materials is known. See the article Aero-Sol-Gel Synthesis of Nanoporous Iron-Oxide Particles: A Potential Oxidizer For Nanoenergetic Materials, by Anand Prakash, Alon V. McCormick and Michael R. Zachariah, Chem. Mater. 2004, 16, 1466-1471, a publication of the American Chemical Society. The article describes the use of nanoparticles of a fuel such as aluminum and a metal oxide oxidizer, which react to liberate a large amount of energy. The high surface area per volume of material engendered by the very small particle sizes is stated to reduce mass-transfer limitations and achieve a chemical-kinetically controlled ignition. The oxidizer particles which are the subject of the invention are said to be in the 100 to 250 nanometer (“nm”) size range.
UK Patent Application 2 049 651 of Brock's Fireworks Limited, Dumfriesshire, Scotland discloses a process for applying a pyrotechnic or explosive composition to a surface by screen-printing the composition in the form of a liquid slurry or paste onto the surface allowing the composition thus obtained to dry and/or harden. It is disclosed that several layers may be applied, preferably, through a coarse mesh screen which allows relatively large solid particles to pass therethrough without becoming clogged. A size range of particles is not mentioned. It is further disclosed that several layers may be applied in the described manner and each layer may be the same or different. A final layer of inert material may be overprinted for purposes of waterproofing or to prevent ignition at the surface and, if desired, flocking may be applied between steps.
U.S. Pat. No. 6,712,917 issued Mar. 30, 2004 to Gash et al and entitled Inorganic Metal Oxide/Organic Polymer Nanocomposites and Methods Thereof discloses a method of producing hybrid inorganic/organic energetic nanocomposites.
U.S. Pat. No. 6,803,244 issued Oct. 12, 2004 to Diener et al and entitled Nanostructured Reactive Substance and Process For Producing the Same discloses a nanostructured reactive substance of, e.g., silicon and an oxidizing agent. The nanometer scale size of the particles, which are initially separated by a barrier layer, is said to permit virtually direct contact between the fuel and the oxidizing agent, once the barrier layer is broken open.
A detailed discussion of thermite mixtures, intermetallic reactants and fuels is contained in the paper Theoretical Energy Release of Thermites, Intermetallics, and Combustible Metals by S. H. Fischer and M. C. Grubelich, of Sandia National Laboratories, Albuquerque, N. Mex. The paper, SAND-98-1176C, was presented at the 24th International Pyrotechnics Seminar, Monterey, Calif. in July, 1998.